setdefault() 方法如何在此反转字典实现中工作？

Question

谁能解释一下在下面的例子中字典“inverse”的赋值是如何发生的？

def invert_dict(d):
  inverse = {}
  for key in d:
    new_key = d[key]
    inverse.setdefault(new_key, []).append(key)
  return inverse

letters_in_word = {"mine": 4, "yours": 5, "ours": 4, "sunday": 6, "friend": 6, "fun": 3, "happy": 5, "beautiful": 8}

print (invert_dict(letters_in_word))

输出当然是正确的：

{8: ['beautiful'], 3: ['fun'], 4: ['mine', 'ours'], 5: ['happy' , 'yours'], 6: ['sunday', 'friend']}

Python 3.x 文档说：

setdefault(key[, default]):

If key is in the dictionary, return its value. If not, insert key with a value of default and return default. default defaults to None.

让我用一个例子来说明我做的和不明白的：

假设 new_key = "快乐"
new_key 的值为 5
setdefault() 被调用，让我们假设 5 已经在“你的”字典中（据我所知，因为字典是无序的，所以不一定是这种情况，但让我们假设）并将return ["yours"] （我的猜测是这里发生了一些稍微不同的事情，所以事实上 "inverse.setdefault(5, [])" 有 returned ["yours"] 就是这样)
append() 被调用并且 ["yours"] --> ["yours", "happy"] - 这就是我们剩下的。

我知道到 4 结束时我错了，因为实际上我们的列表已分配给键“5”。我不明白的是那件事发生的时间点 - 似乎我们只是 returned 并且真的应该分配给：

inverse[new_key] = inverse.setdefault(new_key, []).append(key)

但是，如果我运行这样的代码，我会得到错误 - 'NoneType' object has no attribute 'append'。

欢迎任何解释 - 我想我一定遗漏了一些关于这两种方法如何相互作用的信息。

P.S。这是我的第一个问题，如果问题 nature/structure 不是 'how things are done around here'，我深表歉意。让我知道如何改进，我会尽力做到！

Answer 1

打印语句是了解程序中发生的事情的一种非常有用且简单的方法：

def invert_dict(d):
    inverse = {}
    for key in d:
        new_key = d[key]
        print('key:', key)
        print('new_key:', new_key)
        print('inverse before:', inverse)
        value = inverse.setdefault(new_key, [])
        print('inverse in the middle:', inverse)
        print('value before:', value)
        value.append(key)
        print('value after:', value)
        print('inverse after:', inverse)
    return inverse

letters_in_word = {"mine": 4, "yours": 5, "ours": 4, "sunday": 6, "friend": 6, "fun": 3, "happy": 5, "beautiful": 8}

print(invert_dict(letters_in_word))

输出：

key: beautiful
new_key: 8
inverse before: {}
inverse in the middle: {8: []}
value before: []
value after: ['beautiful']
inverse after: {8: ['beautiful']}
key: yours
new_key: 5
inverse before: {8: ['beautiful']}
inverse in the middle: {8: ['beautiful'], 5: []}
value before: []
value after: ['yours']
inverse after: {8: ['beautiful'], 5: ['yours']}
key: ours
new_key: 4
inverse before: {8: ['beautiful'], 5: ['yours']}
inverse in the middle: {8: ['beautiful'], 4: [], 5: ['yours']}
value before: []
value after: ['ours']
inverse after: {8: ['beautiful'], 4: ['ours'], 5: ['yours']}
key: sunday
new_key: 6
inverse before: {8: ['beautiful'], 4: ['ours'], 5: ['yours']}
inverse in the middle: {8: ['beautiful'], 4: ['ours'], 5: ['yours'], 6: []}
value before: []
value after: ['sunday']
inverse after: {8: ['beautiful'], 4: ['ours'], 5: ['yours'], 6: ['sunday']}
key: happy
new_key: 5
inverse before: {8: ['beautiful'], 4: ['ours'], 5: ['yours'], 6: ['sunday']}
inverse in the middle: {8: ['beautiful'], 4: ['ours'], 5: ['yours'], 6: ['sunday']}
value before: ['yours']
value after: ['yours', 'happy']
inverse after: {8: ['beautiful'], 4: ['ours'], 5: ['yours', 'happy'], 6: ['sunday']}
key: fun
new_key: 3
inverse before: {8: ['beautiful'], 4: ['ours'], 5: ['yours', 'happy'], 6: ['sunday']}
inverse in the middle: {8: ['beautiful'], 3: [], 4: ['ours'], 5: ['yours', 'happy'], 6: ['sunday']}
value before: []
value after: ['fun']
inverse after: {8: ['beautiful'], 3: ['fun'], 4: ['ours'], 5: ['yours', 'happy'], 6: ['sunday']}
key: mine
new_key: 4
inverse before: {8: ['beautiful'], 3: ['fun'], 4: ['ours'], 5: ['yours', 'happy'], 6: ['sunday']}
inverse in the middle: {8: ['beautiful'], 3: ['fun'], 4: ['ours'], 5: ['yours', 'happy'], 6: ['sunday']}
value before: ['ours']
value after: ['ours', 'mine']
inverse after: {8: ['beautiful'], 3: ['fun'], 4: ['ours', 'mine'], 5: ['yours', 'happy'], 6: ['sunday']}
key: friend
new_key: 6
inverse before: {8: ['beautiful'], 3: ['fun'], 4: ['ours', 'mine'], 5: ['yours', 'happy'], 6: ['sunday']}
inverse in the middle: {8: ['beautiful'], 3: ['fun'], 4: ['ours', 'mine'], 5: ['yours', 'happy'], 6: ['sunday']}
value before: ['sunday']
value after: ['sunday', 'friend']
inverse after: {8: ['beautiful'], 3: ['fun'], 4: ['ours', 'mine'], 5: ['yours', 'happy'], 6: ['sunday', 'friend']}
{8: ['beautiful'], 3: ['fun'], 4: ['ours', 'mine'], 5: ['yours', 'happy'], 6: ['sunday', 'friend']}

一个很好的调试器也非常有用，例如 PyCharm 中的调试器。试试看。

setdefault() 方法如何在此反转字典实现中工作？

How is setdefault() method working in this invert dictionary implementation?

python

methods

setdefault